1. Field of the Invention
The invention generally relates to data communication systems such as WLAN (Wireless Local Area Network) systems, and in particular to phase error corrected receivers in such systems.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimising the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.1 b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
In particular, the 802.11b standard is specified for non-coherent receivers. However, the receiver sensitivity can be increased when using a coherent receiver which however requires sufficient phase error correction. Differential demodulation for differentially modulated signals as defined in the 802.11b standard is insensitive to a constant phase offset but will be heavily effected by an abrupt change in phase. Abrupt phase changes are typically produced when performing phase correction by means of an equalizer.                An example of a common arrangement in a wireless LAN receiver is illustrated in FIG. 1. A timing error and frequency error correction unit 102 receives a digitized baseband signal as an input and provides a timing error corrected and frequency error corrected output signal to an equalization unit 103. The equalization unit 103 performs a phase error correction. The finally output coherent signal of the equalization unit 103 however includes a phase shift at a certain point in time and thus becomes difficult to be correctly differentially demodulated.        